When selecting a wind turbine for a given operating location, consideration is given to characteristics of the site such as the complexities of the site terrain and the average wind conditions. The turbines chosen can ideally operate at rated power for as much of the time as possible. However, in practice, wind speeds are variable and the turbine must be able to cope with a wide variety of wind speeds. At lower wind speeds the power output will either be zero, if there is negligible wind, or below rated power. Once the wind speed increases to above that required for generation of rated power the turbine will protect itself from damage, for example, by varying the pitch of the blades to reduce the power extracted from the wind. In extreme cases the turbine may shut down or yaw out the wind to prevent catastrophic damage. However, an emergency shutdown or yaw procedure takes time and, in some circumstances may not be able to prevent severe damage to turbine components from occurring.
When designing wind turbines it is desirable to maximise the length of the blades and generally to minimise the weight of components. However, this process is a balance between reduction in the cost of generating electricity and the integrity of the wind turbine; the turbine must be designed to be able to withstand the most severe wind conditions to which it will be exposed.
One partially problematic wind condition is an extreme gust. Such a gust may occur only once every year or every few years, but has the potential to cause serious damage to the wind turbine. A particular gust profile used by designers is the so-called ‘Mexican hat’ gust in which the incident wind speed drops just before the gust hits the turbine. The drop in speed is detected at the turbine which may pitch the blades in response to increase the power generated. When the extreme gust then hits the turbine rotor, the blades are pitched at an inappropriate angle which greatly increases the loading on the blades and so the likelihood of serious damage to the turbine components. The Mexican hat extreme gust condition is described in International Standard IEC 64100-1, Third Edition at S.6.3.2.2. International Standard IEC 61400-1 defines a gust as a temporary change in wind speed (definitions 3.20) and extreme wind speed as value of the highest wind speed, averaged over t s with an annual probability of exceedance of 1/N (“recurrence period”: N years). A note to this definition states that in the Standard recurrence periods of N=50 years and N=1 year and averaging time intervals of t=3 and =10 min are used, however, the turbine is designed using extreme wind speeds for design load cases.
Section 6.3.2.2 defines an Extreme Operating Gust (EOG) as:
The hub height gust magnitude Vgust shall be given for the standard wind turbine classes by the following relationship:Vgust=Min{1.35*(Ve1−Vhub);3.3*(σ1/(1+0.1*(D/λ1)))}
where σ1 is the representative value of the turbulence standard deviation;
λ1 is the turbulence scale parameter;
D is the rotor diameter.
Other extreme conditions which designers of wind turbines must take into account include extreme changes in wind direction, extreme wind shear, extreme turbulence and an extreme coherent gust with direction change.
Because of the need to design wind turbines to survive these extreme conditions, turbines are effectively over engineered for normal operating conditions. We have appreciated that if the effects of these extreme operating conditions could be mitigated, the amount of material used in turbine blades in particular would be reduced decreasing manufacturing costs. Alternatively the blades could be made larger which may increase energy capture at low wind speeds and enable a greater rated power to be achieved.